1. Technical Field
The present invention relates to a process for preparing a homogeneous porous layer of biocompatible polymer, the said process comprising, inter alia, a step of lyophilization of a viscous solution or suspension of the said polymer.
2. Description of Related Art
A surgical implant for repairing the dura mater is often in the form of a relatively flat slab formed from a porous layer obtained from the lyophilization of a solution comprising biopolymers.
Such a surgical implant should have good conformability in order to be able to take on the exact shape of the dura mater. It should also have on its thickness the most uniform possible density and porosity in order to be efficient once implanted.
All the parameters that are liable to have an influence on the final homogeneity of a lyophilized product cannot be totally mastered and controlled in the course of a lyophilization process. Specifically, for example, the temperature in the product may vary according to the thickness of the product. Similarly, the nature of the mould initially receiving the viscous composition may also have an influence on the lyophilization kinetics. Similarly, the difference in temperature at the lower edge of the mould and on the upper face of the product, simply in contact with air, may also have consequences on the homogeneity of the lyophilized product.
In point of fact, it has been observed that many products, all in the form of a lyophilized layer, have a non-uniform density and porosity: in particular, the upper part of the lyophilized layer, that which is in contact with air during the lyophilization process, has a density that is much higher than the rest of the layer. On the other hand, this upper part has a much lower porosity than the rest of the layer. In certain cases, the porosity at the surface of the layer is nonexistent. Finally, in the case where surface porosity exists, the surface pores are not connected to the pores of the inner porosity of the layer.
Such a phenomenon affects the properties of the products obtained: in particular, the capacity of the product to absorb fluids rapidly is compromised. Consequently, the conformability of the product is degraded, and its handling is made more difficult. This represents major drawbacks when these products are implanted into the human body, for example in the form of surgical implants for the purpose of repairing a defect in the dura mater.
It would be desirable to perform a process for preparing porous layers which would avoid this phenomenon and which would make it possible to make implants, in particular for repairing the dura mater, obtained by lyophilization of viscous polymeric compositions, and having the most uniform possible density and porosity over the entire thickness of the layer, and in particular having a surface porosity in which the pores are connected to the pores of the inner porosity of the layer.